So far, a cap, which is what might be called an “overcap”, is attached to a liquid drug-expelling part in order to prevent vapor or moisture from entering into the liquid drug-expelling part when a syringe is sterilized by heating as is disclosed in the Patent Document 1.
A cap of this kind (referred to as a “nozzle cap” in the Patent Document 1) comprises a top surface portion that makes an external form, an outer tubular part that is formed below the outer periphery thereof to tightly seal a luer lock part and an inner tubular part that is formed below the top surface part on the inside thereof to tightly seal a luer nozzle. Then, as is disclosed in the Patent Document 1, it is customary that the length of the outer tubular part that tightly seals the luer lock part is longer than the length of the inner tubular part that tightly seals the luer nozzle part.
If the outer tubular part that tightly seals this luer lock part is long so that the space that exists in the inside thereof is large, then when the cap is removed from the liquid drug-expelling part, reduced pressure is created in the space that exists in the inside thereof so that reduced pressure causes leaking of a liquid drug from the syringe. Further, if the outer tubular part is long and the part that closely contacts with the luer lock part is large, then it is inconvenient for a user to remove the cap from the liquid drug-expelling part with ease. Also, if adhesion of the close contact part increases with a time, then it becomes more difficult for the user to remove the cap from the liquid drug-expelling part.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-210008